Linux 网络编程完全指南

# 查看connect的文档
man connect
# 查看man的文档
[root@localhost ~]# man man
   ## 无关的部分，省略...
   The table below shows the section numbers of the manual followed by the types of pages they contain.

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]

   A manual page consists of several sections.

socket(AF_INET, SOCK_STREAM| SOCK_NONBLOCK, 0);

// 普通设置调用fcntl
int flags = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

Since Linux 2.6.27, the type argument serves a second purpose: in addition to specifying a socket type, it may include the bitwise OR of any of the following values, to modify the behavior of socket():

SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on the open file description (see open(2)) referred to by the new file descriptor.  Using this flag saves extra calls to fcntl(2) to achieve the same result.

SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor.  See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.

ulimit -a  # 查看所有的选项，当前回话的
ulimit -c unlimited # 修改corefile的size
ulimit -n 65535 # 修改打开的fd最大数量

#include <iostream>
#include <unistd.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/fcntl.h>
#include <arpa/inet.h>
#include <sys/time.h>
#include <sys/types.h>
// 设置非阻塞
void SetNonBlocking(int fd) {
    int oldFlags = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, oldFlags | O_NONBLOCK);
}
int asyncConnect(const char *hostname, int port, int timeout) {
    // 1.创建socket设置非阻塞
    int sockfd = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0); //一步到位设置非阻塞

    // 2.连接服务器
    struct sockaddr_in serveraddr;
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_port = htons(port);
    inet_pton(AF_INET, hostname, &serveraddr.sin_addr);
    
    for (;;) {
        int ret = connect(sockfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
        if (ret == 0) {
            std::cout << "connect to server successfully" << std::endl;
            return sockfd;
        } else if (ret == -1) {
            // connect 被信号打断，重试connect
            if (errno == EINTR) continue;
            else if (errno == EINPROGRESS) {
                //连接正在进行中
                break;
            }else {
                // connect出错
                close(sockfd);
                return -1;
            }
            
        } 
    }

    fd_set writeset;
    FD_ZERO(&writeset);
    FD_SET(sockfd, &writeset);
    struct timeval tv;
    tv.tv_sec = timeout;
    tv.tv_usec = 0;
    // 3.调用select函数判断socket是否可写
    if (select(sockfd + 1, NULL, &writeset, NULL, &tv) != 1) {
        // 超时或者出错,具体判断select返回值， =0超时返回，<0出错
        std::cout << "select connect to server error" << std::endl;
        close(sockfd);
        return -1;
    }
    // 4.调用getsockopt检测此时sockfd是否出错
    int err;
    socklen_t len = sizeof(err);
    if (getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &err, &len) < 0) {
        // 出错
        perror("getsockopt");
        close(sockfd);
        return -1;
    }

    if (err == 0) {
        std::cout << "connect to server successfully" << std::endl;
        return sockfd;
    } else {
        std::cout << "connect to server error" << std::endl;
        close(sockfd);
        return -1;
    }
}

如果应用程序不关心bind绑定的ip地址，可以使用INADDR_ANY(如果是IPv6，则对应in6addr_any)，
这样底层的（协议栈）服务会自动选择一个合适的ip地址，这样使在一个有多个网卡机器上选择ip地址问题变得简单。

全连接队列(已完成队列)：三次握手完成等待accept的连接
半连接队列(SYN队列)：三次握手未完成等待三次握手建立的连接，三次握手成功后从半连接队列移动到全连接队列中。
backlog是两队列队列的最大长度。
Linux 内核中，backlog 参数影响 sk_accept_queue 和 sk_syn_queue，分别对应全连接队列和半连接队列。